1997 Other Volkswagen Models abs/brake light comes on and th

My son installled an after factory radio in my 1997 Volkswagen Cabrio. (with a wiring harness) and then my abs/brake light comes on and my headlights/windshield wipers/fan blower all goes out. It happens at odd times. As soon as the abs/brake light goes off everything works fine. He has tried several different things (like wiring the radio into the egnition?). Some one told us that the alternater could be bad. So we removed it and took it in to Auto Zone and the tested it and they said it was fine!!

Hopefully you could suggest somethig else to try.

sincerly, Debbie Burris