There is a TSB/fix for this. This TSB is the most comon thing that goes wrong with the seat heater. However, it sometimes is the main relay on the underside/side of the seat.
Do you hear the relay kick on when you flip the switch? If so, it's probably not that.
The fix is pretty easy if your inclined to doing miner sodering. In the TSB they list the procedure on how to check the heaters in the seat cushon. http://home.sc.rr.com/janet/TSB/08-03-99.htm
The kits are about $15 a piece at the dealer.
ZJ 98 5.9 LTD. Lots of mods. 7" T&T's Long Arm Lift ,BFG KM2"s,35X12.50x16's,4:56's, Vanco brake upgrade, M1intake,Holley52mmTB,MikeLeach Headers,, Arb'ed HP D30,Aussie Locked rear,Procomp MX6 Shocks,Arb front,RearTR bumpers,ECT.ECT.