SRS light on '95 E420

[callard]

I've had the car for over six years. About a year ago the SRS light started coming on intermittently while driving. Eventually it stayed on most of the time and now it is always on. Recently, I took it to an MB tech to read the code or reset the light. He wasn't able to communicate with the SRS computer while all other computer queries/functions worked fine.
I have since acquired two separate SRS computers (MB 000 820 72 26, Bosch 0 285 001 078) and installed under the ashtray. In each case, the MB tech's computer was not able to communicate with the replacement SRS module much less get any codes from it. When the SRS is unplugged, the dash light is out. When plugged in, the light stays on with any of the three modules. I've tested the battery and have 14.6 volts with the engine running and I've disconnected the battery cables for 30 minutes with the same results. I'm at a loss for further tests. Any ideas?
Thanks,
Chuck