This is a common problem with older seat heating elements. As they get older the resistance gets higher and the module shuts them off. Sometimes as mentioned it is just a bad connection. On my Suburban that module under the drivers seat controls a ton of features including the pedal adjustments, so anything that isn't working may affect the seats from being heated.
Steven
2004 Marauder Black, loaded
Vortec S/C
4.10s
3500 PI Stall
Kook's headers
476.9 rwhp 385 lbs/tq
KVR's, QA1's, M&Z control arms, rear sway bar
Lidio tune
2004 Suburban Dark Charcoal
1970 W-30 442 convertible twilight Blue