Probably just the elements in the seats. Inexpensive check would be to unbolt the seat , unplug it and place a bolt meter on the leads. If you need to change the elements there are several to choose from on EBay. The downside is that they will not get as hot. The upside is that they will not burn out as quickly. Complexity level approx 3/10. Will take about 3 hours start to finish. You will need to pull up the leather and place new pads