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Originally Posted by
mad smart person
If you're really keen, assuming both bulbs are the same, measure the current each bulb draws - it should be the same, if not, there's a restriction in current flow in the car's wiring
Simply checking that there is 12volts across each bulb at the connector would have precisely the same effect , and you wouldn’t have to cut into the wiring either.
Any ‘restriction’ in current flow as you say, would simply be down to a excess volt drop in the rest of the supply route back to the battery on both the feed and ground return.
If there is substantial (and unusual) circuit resistance, then that would result in a ‘lower than battery voltage’ volt drop across the bulb, so you will be able to check the integrity of both feed and earth return simply by checking volt drop across the bulb. (It should be ~battery volts)
You're so money supermarket and you don't even know it!