R129 500 SL 1991
If you’re talking about the relay in the engine module compartment, that’s the OVP.It is about an inch square with a clear plastic top. there are two fuses in it that also can have an effect on the ABS/ASR...
Resistance can build up in the contacts, wreaking havoc with ASR/ABS operation.
MB sequence of diagnosis of ASR/ABS light on with engine in limp home is to check the throttle linkage first (as you quite rightly pointed out), and then the throttle potentiometer.
Insight into effect of OVP is here