Retrostu

My mate asked me to take a look at his car for him, it's a 85' 1600 cabriolet with s1 body kit ( i know its not a turbo but didnt where else to get the best knowledge base).

The heater box motor was not working so i tried his switch on my S1 and all ok i then took my relay (small yellow one nearest the drivers side wing) out and tried in his and the blower started working all ok.
I then used another spare relay as i wanted my original back the spare he could keep same as the other in the headlight relay slot all ok lights worked fine but on high beam/flash passing light the brown/yellow wire start to frazzle and burn out.
I then changed the headlight relay back to the original one he had and it still did it.
I feel really guilty although i have not really done anything.
(blower still work though)

I hope this dose not sound toooooooooooooo confusing.
please any help advice gratefully received.