Heater fan will not run

The ses lite needs voltage and ground. Voltage goes hot when you turn the key on. The computer controls the ground circuit. I can't rule out, your instrument cluster may have a problem? The ground circuit could be shorted to ground before it reaches the computer. Or the computer has a problem? Just my opinion.

most relays for this are under the hood in the fuse box it is usually on the pass side just on the wheel well could be on drivers side also but it should be there ,

no diagram ,sorry , but usually located in the fuse/relay box in the engine compartment and the cover will have list of which is which

Maybe it helps

https://www.autogenius.info/category/oldsmobile/oldsmobile-regency/

try autozone

if you haven't got your blower fan fixed yet, im sure it to be the same as mine, the blower motor relay which is out on the firewall under the big fuse block. take plastic cover off over the fuses and move the fuse block out of the way and you will see it staight under that. should be two screws holding it on.

fusebox

Review this Wiring Schematic about Olds 98 ('89, '90 & '91):
1989 1
1989 2

1990 1
1990 2

1991 1
1991 2

I really hope help you with this (remember rated this help). Good luck.

There is a fusible link for the blower motor. Check that. Don't know where it is. Look up the wiring diagram at autozone.com. Also check the fuse in the fuse box. Could just be that.

replace the blower speed control module located near the blower motor