Cabin access through firewall?

[ockhamsrazor]

Am fitting aftermarket front parking sensors to 06 V8V (LHD), and need to route a wire from the engine bay through the firewall into the cabin. Cursory look under bonnet does not reveal any obvious access holes, but they must be there! I'd rather not drill a new one, so can Rick or anyone direct me to an existing point of entry (as it were)?

Tia.