Light problems on my RS100

I just got myself a little RS100B and I know there are a few problems with the wiring. I have not gotten a chance to pull the headlight out yet cause the last owner stripped out both bolts in front (adjustment and open), but I'm presuming I'm having magneto problems because my brake light bulb works fine but I only get brake light and not tail light when I turn the key to accessories(headlight/tail light). Where should I start to check on the tail light problem?

Thanks in advance.

Matt