On my 1991 911 C2 there is a chronic problem with the ABS and seatbelt light come on and stay on. When the lights, using the hammer, are reset they will stay off for just a short time 50 t0 150 miles. The tech manuals don't discuss this at all. In the Adrian Streather "Enthusaiast's Companion" he recommends that the leads from the connector to the lights on the instrument be checked for proper separation which was done with no change. All grounds in the circuit were checked and renewed with no change. I have had this car since and this problem has been with it for at least 14 years.
Any ideas would be helpful.