voltage differances in headlight bucket

[GS1100ez]

on my ez i added a cig lighter for a gps charger
i am thinking about wiring it into the headlight to save some time from removing everything to get to the battery

i took the headlight out and took my meter to some things in there

engine not running, key on
at the headlight i get 12.6v
there is a spare wire hanging out in there, which isnt plugged into anything, which is key powered which is only getting 10.5v

with it running the spare wire goes up to 12.6v

why would i have 2 hots which have differnt voltages?
is there that much wiring to cause voltage drop like that in there?

does anyone know what this wire is too? or is it spare on purpose?

82 gs1100ez

[tkent02]

I think it's a spare, it would have gone to the second filament in the turn signals if you had those. Not sure. Should be good to use for the GPS.

[Nessism]

The headlamp circuit most likely uses a larger gauge wire, and may have a more direct current path than that accessory wire, thus less resistance. At any rate, that accessory wire should do fine for you gps I should think.