Does the fuse panel have lights that illuminate when the fuses are blown? Those will cause stray voltages on the output terminals with the fuse pulled when measured with a voltmeter. If the dealer isn't familiar with that, I would start thinking about going to another dealer for servicing....
(Those little lights are nothing more than LEDs with appropriate dropping resistors in parallel with the fuse. If the fuse is not blown, it shorts out the LED and it does not light. If the fuse is blown and there is some load on the circuit, a handful of milliamps can flow through the LED, limited by the resistor and the impedance of the load, and it illuminates. If there's no load on the circuit, then the LED won't illuminate but the load side of the fuse connection will float up to about 12V.)
