Multimeter and for the quick check a test lamp, OBD reader, jumper box, jumper cables - I think that's all I have on board in terms of electrical tools.
In addition, fuses - a tip: please check exactly which fuses you need for your rig and take them with you - spare bulbs, vehicle-specific relays and always one or two standard relays and a universal on / off switch, various cables, crimp connectors, soldering connectors, heat shrink tubing, electrical tape (so you can improvise quite a bit).
On long trips I also have some of the most important sensors with me (crankshaft sensor, mass air flow sensor, camshaft sensor, alternator regulator and these things).
What is of course also essential is to have documentation of the vehicle, i.e. where is which fuse and which relay and ideally a complete circuit diagram (here it depends of course on what skills you have yourself, but a circuit diagram can also be useful in a garage if the mechanic does not know the brand/ type of rig).