Yellow/Black Wire - MK1 series 2 Fusebox

Hi guys, havent posted in a while as have been too busy!

Finally got back to trying to tackle some wiring on my mk1 > 16v KR Conversion.

The wire im so confused about is as stated a black/yellow wire which comes from Block C pin 13 on the loom which in theory should be for the dual horns. When this wire recieves a 12v supply from the battery it seems to be priming the fuel pump. where should i connect this!? and if its not for the horns which wire is! have tried following notes on guides but to no success! if it helps for any additional info my car originally had MFA wiring (dont know if that makes a difference to wire colours on fusebox???

Please help!

thanks!

this wire is an ign live output to the horns, which is fed from the x-over relay on the fusebox. so you shouldnt be putting 12v to it