86 300sl mystery wire
 

I have an 86 300sl that has a black wire with a yellow stripe next to a brown [ground] wire next to my battery located under the hood on the passenger side. It has power going to it when the key is on. Anybody know what it's for? I also have a battery drain issue, I think. When I connect a test light between the neg. post and the neg. cable not connected to the battery the light is on although not real bright, is that normal? Thanks. Greg.

do a draw test with an ampmeter
Connect the black wire to the com input on the multimeter and the red wire to the 10A or 20A input on the multimeter.
If the ampmeter is reading over 25-50 milliamps, something is using too much battery power. remove fuses one at a time to find the draw area
connect meter to the mystry electric line, do the same with the fuses , I suspect power antenna option?

battery drainage
 

I found out that the driver's side sunvisor vanity light was staying on after the key was out. So I disconnected it. I also noticed that my driver's side low beam headlight was intermittent even after I replaced it, somehow the side marker which wasn't working anyway [ I checked the bulb] was somehow affecting it so I pulled out the side marker bulb and now the headlight stays on like it should, go figure. I'M NOT AN ELECTRICIAN. Greg.

Converted to US headlights?

yes they were before I got it.