Greetings! First post in the forum today.
I have the 88° C thermostat noted above on my 2003 E39, and it throws a code about the heater about every fourth start. It has Dinan software so that may contribute. The E39 forums discuss adding a resistor (sorry, don't recall part number or where I bought it) but the infrequent code issue persists.
I just found this alternative at ECS; it has the heater connector. Bought one to try in my 2002 E53 4.6iS:
https://www.ecstuning.com/b-bremmen-...31436386~ber~/
I'll likely put one of these in the E39 too if it's trouble-free in the E53.
Larry