I recently attempted to install HF in my 2003 Chevy Avalanche. The rig is a ft897 and the antenna is a Hustler mo4 mount with resonators for 40 and 80 meters.
 
 The problem I am having is on 75 mtrs.  I am RFing the black box (brain) of the truck. Every time I use a power level greater than 20 watts on 75 mtrs the trucks gauges, seat belt alarm, door ajar alarm and check 4wd lights and bells come on.   When using it on 40 mtrs, it does great.  

  I am assuming at this point that for some reason the frequencies from 3.890 to 3.930 are such that they swamp the black box with rf.  This is the only frequency range I tested on 75 mtrs since the hustler was resonant at the center and rose to 2 to 1 at the edges.

   Again, 40 works great and I recieved great reports on it.  I have no other resonators to try so I cant say whether it has problems on any other band,  

   I have tried changing the antenna cable routing and also tried a chunk of rg-8 and still had the same results.  
 
    The power cables and antenna cables exit the cab of the truck through the rubber gromet that also has the fuse/relay bundle running through it.  

    I tried a isolated source for power i.e a marine battery in the cab... but still same results.

    I put a telewave dummy load on the rig and could key down 100 watts with no problems, I even tried the dummy load on a long piece of rg8x and keyed down and ran the dummy load all over the cab and engine compartment hoping to spot the affected area.  

    Nothing happened, the rig perked along at 100 watts and the vehicle had no problem.  So I need the help of some more experianced mobileers.

    Any and all suggestions cheerfully appreciated.

     Tnx in advance  de AA9YP