2000 E39 ABS/DSC question

Greetings,
I have a 2000 E39 (528i)
Two years ago my ABS, DSC and speedometer became inop. I was told the ABS module went bad.
I sent it to a place that repairs them. When I recieved it back, I put it
back in and everything came back to life immediately.
Six months later the ABS and DSC stopped working. The Speedometer still works.
Thinking the ABS module died on me again, I sent it in to be checked. It was tested to be ok.
Do you think it could be a bad speed sensor?
I know there is a front and a rear. Is there any simple way of finding out which one is bad?
My car is due for inspection by the end of the month and a friend will replace the rotors and brake pads. Would be nice if we could find the bad speed sensor (if there is one)
Comments please..
Jeff
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There are up to 5 speed sensors. There is one on the diff that tells the car how fast it is going, and one on each of the tires to tell the car which tire is locked.
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It is probably the ABS sensor on one wheel - a code read will tell which one.
The GPS will be sporadic as well.

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