Check the wiring to and from the ammeter and the thick red and black wires going into the bulkhead connector. Peel back a little wiring covering and check. If those wiring starts to toast it will do really weird things. I could not turn off my side marker lights but I could dim them from the dash dimmer switch last time it happened to me.

Those wires can start to melt from high amperage flow either by electrical loads or overcharging alternators. When they do they melt into other wires and provide current to circuits that could be anything from wipers to hazards and act really freaky.

Good Luck