use decent cable - will not get interference as there is no signal wires there!! - or drill a new hole but do not drill it vertically as you may well cause a leak even with a grommet

the main loom from engine bay to dash board will be in a big grommet with enough space round it for a new hole - push a bit of coathanger thru it & attach power cable to other end once thru then pull it back the other way so cable feeds into cabin!!

good luck