How to use a relay to "lift" 12v ground of a two 12v battery system
You are imagining a problem where there is none and likely to create headaches for yourself. The solution is to wire a multicore cable with regular trailer plug on it along to the back of your trailer, ground the negative to the trailer chassis and wire the cores to the lights, disconnecting and insulating the original tail lamp wiring.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. It should be clear from the schematic that the only connection between the two systems is the common battery negative chassis connection. Without a positive connection between the two systems no current can flow between them.

The result is a system that will work on any tractor unit, doesn't rely on trailer battery condition, is simple, standard and reliable, and can be maintained by any decent mechanic.