Strapping two amps opens pico fuse of HU?

I have two SAZ-3500's, wire them up according to the diagram for strapping. As soon as I apply power the pico fuse blows in the HU. I can run them individually without issue. I notice a slight thump with one amp as I adjust the volume.

I can only think I have a gnd issue in the one amp. I don't measure any dc on the RCA gnd. I can run them fine on my workbench with my spare HU.
After repairing the head unit, insert a ground loop isolator between the amp and the head unit and look for AC voltage on the amp RCA shields when the amps are strapped.

I'm assuming that you have the negative speaker terminals connected together as is shown in the manual.

Yes the negative terminals are tied together. The BR OUT rca goes to the BR IN on the slave amp. I find it interesting that it won't protect on my bench. I'm using a Pioneer HU, everything is tied off the same supply. So the gnd point is the same, only thing missing is the second amp I have in my car.

I see no AC voltage on the rca gnd terminals, what is causing the issue in the car? Same problem the owner of these is having.
The problem may be intermittent. Try twisting the transformers and push on various parts of the board to see if that ever causes AC or DC on the RCA shields.

Do this near the RCA jacks as well. There could be a resistor that's shorting to a trace under it.

You should also check for excessive DC offset on the op-amps near the RCAs. That could be indicative of a problem in that area.

Is there anything different about the install in the vehicle (like the remote being plugged in where it's not on the bench)?