a very cheap faultfinding tester

you will need a plastic box big enough to hold 2 lampholders and lamps 6 / 12 / 24 volt, whatever voltage you are testing.
you will need 2 diodes, and 2 crocodile clips (large) and 1 probe, with 3 lenghs of cable. connect and solder (centre cable (yellow)) to both lamps one (orange lens) and one (red lens), solder 2 diodes anode to kathode faceing away from the the two lamps, solder 2 cables (all long enough) to each end of lamp holders. the orange lamp cable (orange cable). the red lamp (red cable), this cable has a probe. this probe will check any bad earths. the yellow cable go's to the battery positive (battery positive diconnected), the orange cable connects to battery lead (to make circuit orange lamp will light). the red probe can be tested by pushing on the negative terminal (battery). you cannot short out any wiring because the lamp and doide (in circuit). if doides reversed will check (positive connections)

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gerald pannier

Great idea except the bulbs will draw enough juice to fry most ECUs.
Both Snap-On and Mac market LED versions. If you are into that get a power probe, it will do what you are suggesting more safely.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.