I usually follow the main wiring loom and then carefully drill a hole thru the firewall beside it and fit a waterproofing grommet with cable skirt (its like a grommet but has a rubber tube you cable tie around the cable to waterproof it).
Of course you must check you aren't going to damage anything when you drill the hole.
2015 5 dr Grand Vitara 2.4 (daily driver)
1971 V8 HG Holden with 383 stroker 9" torq lok and TH350 trans (the toy)
Mitzi RVR 2.0 (not one of my better buys)
the 2013 3 dr GV is now the off road only toy
"Dogs feel very strongly that they should always go with you in the car, in case the need should arise for them to bark violently at nothing right in your ear."