[Hummin Cummins]

I was coming to work today and when I left the house, I got about 2 miles up the road and the check engine light came on and the voltage was low. I went to my neighbors auto elect. shop and he tested my alternator and it was fine. We are thinking the grid heaters stuck. I left and went to work, voltage stayed in the 14+ range. Started trouble shooting the relays according to the manual and found both would click, but only one was closing the circut on the ohm meter. Figured the heaters were stuck and relay is junk now. All my testing was done with both batt. ground cables unhooked as per the manual. I found power at the intake heaters with a test light with the key off and batterys unhooked on the neg. side. Tested one battery and found power coming through the neg. batt. terminal? Tested the other and all was normal.
What did I fry, and is it safe to drive?

Stuck at work and dont wanna catch my truck on fire, please help.

Chris