Installed in front seats of 2016 Wrangler JKU under Bartact seat covers. Install was straight forward and included wiring was sufficient to mount switches at front of center console(forward of shifters). These heat up quickly, I usually switch to low setting after about 10 minutes. I included pictures from a FLIR infrared imager that shows the heaters off, then on for 5 minutes with an outside temperature of 3*F.

You are most likely going to need hog-ring pliers and hog rings if you are going to install yourself. I found a hog-ring kit at Home Depot for $10. The rings were a little large but worked well. I would recommend looking online and finding upholstery specific sized rings.
I installed the heaters in my 2008 Saturn outlook. I took my time and spent the better part of a day doing it with no hang ups. There's plenty of diy vids on YouTube for reference. If I had to do it again it would probably take me 3-4 hours now that I have the experience.
Now that the cold is here these heaters work Great! The warm up in about a minute and are very comfortable. My installation looks like factory except for the style of the rocker switches.
The instructions call for voltage supply from a "key on" accessory circuit. I spent the most time on this project looking for one in my Saturn but could not find one. So I ended up having to wire directly into the fuse panel. And unfortunately all the circuits on the panel are live before you put the key in the ignition. So I have to be diligent and make sure the heaters are switched off before I turn the car off. Every car is going to be different so I'm sure this problem will not apply to most.
Overall the product is great and works as described.
My only negative is that the LEDs on the rocker switches burnt out a week after I installed them. I contacted the seller and they told me they would send me replacements after I posted my review. I will update my review after I receive the replacements.

Just installed it in a 2017 Jeep Renegade and it works great. No complaints and the harness was long enough to run right up to the fuse box. All 4 pads are working well.

After a week I can say the high setting gets real hot, low is just warm.

Also looking through some of the low ratings (as I did before I purchased it) I think people are not testing the system correctly. I think the chance of having one pad broken is possible, but having all 4? Probably astronomical unless everyone over a consistent period got 4 bad ones, i.e. a major manufacturing problem in an entire batch, and the comments don't reflect that,

My theory is people are testing it improperly when not installed thus thinking all 4 pads are bad, many responses just say "all 4 pads don't work" with no explaination. If you test this while not installed, and with no heat sink (when installed in your car, the thick foam seat padding is a big heat sink) the thermostat in the pad will quickly hit the trigger temperature and shut the pad off very quickly. You need to either put something cold around the thermostat, or place the whole pad in a representative heat sink like the seat padding, or it will shut off almost immediately. I would just put a cold pack on the thermostat in the pad if testing if before it is installed. Just be careful not to leave it alone as it will heat up very quick. On a 28 degree day this past week within 2 minutes the seat was already very warm.

2016 RAM 2500HD. Truck did not come with heated seats. Great kit for adding in. Since my Ram did not have the AC outlet plug on the dash, I was able to mount my switches in that panel for a factory look. Requires 7/8" hole. Use a stepper bit. Once you get to 3/4", check fitment of switches. Slowly increase 1/8" at a time until it fits nicely. I found it was 7/8". Will be ordering another set to replace factory heater pads in a Toyota Highlander.
If you do not want to mess with taking apart Hog Rings and installing new ones, then work from the back side. Most seats there is a seam about or forward of your groin. Those are usually installed with Hog Rings. In the RAM I did not need heat all the way up to my knees. The pad has markings for where you can cut and it fit perfectly up to that first seam (looking back to front). And also for the lumbar(back), same thing. I did not have to remove the seat from the vehicle. Just slid them up under from the bottom/back, carefully pulled back adhesive and finished install. Ran wiring up under the console, dash, under carpet and out through firewall. The relays with the heater pads do not have enough length for them to install under the hood. They will probably be under the dash close to the switches or under your seat. Depending how big your vehicle is.
Under the hood I already had a 30 amp relay for accessories that works while key is on. I tied the main feeds to that relay on the positive and battery to negative.
In the Toyota, I will be attempting to wire into the existing wiring/relay system for the OEM heated seat pads with these aftermarket. Will try to post on how to do that as there is not much info.
Getting the covers off or partially off are not difficult. Requires patience. The C or Z channel clips, connect to metal seat frame. Careful as they are sharp. If you are not handy with tools or basic understanding of DC electric work it may be challenging. Overall its not that hard if you are familiar with turning a wrench, pulling out plastic kick and dash panels and understand basically how to wire up a relay.
For both seats, seat and back, I think just to install the pads took me a total of an hour. That included removing seat cover clips, installing pad, closing it all back up. The wiring and installing the switches took me two hours. Tear down to finish. I have done a ton of electrical wiring and upholstery in cars. If that gives you an idea of time frame. Used to install electronics for EMS.
The final came out looking factory with no exposed wiring.

This is a fairly complicated solution to a common situation. I purchased the OEM type seat heaters for my suv because I prefer a factory look to accessories I install on my vehicle. This comes with enough materials to do 2 seats. I rarely have passengers, so it's a bit overkill, but just "nice to have" on front passenger seat.
However, for a slightly lower price and a LOT LESS complexity, I purchased a heated seat cover for my mom. She just plugs in the cigarette lighter and...done. Plus she has an additional protector on her seat, and it's removable during summer if desired.
So reflect a bit on your situation before deciding which approach is best for you.
If you want/need to do BOTH seats and don't mind getting your hands dirty, then this is probly the better solution for you.