What's the wire for ?

[Wingnut]

Just fitting a brand new loom to the money pit and can't figure out what the blue wire with red tracer is for that runs in roof to rear of bus. It forks off the small loom from the interior light

 

[who?1]

Rear interior light or heated rear window??

 

[Wingnut]

I thought heated rear window but that should be a thick black one which the new loom doesn't have so will have to use the old one (luckily not chopped, hacked, twisted and insulated with Sellotape like the rest) so must be the rear light. Thanks pal