Look into getting a wire gland to mount in the fire wall.
https://www.google.com/search?q=wire...w=1290&bih=768
That with a little silicone should keep it sealed and protect the wires through the hole. I ran an 8g wire through my factory harness on the driver side to power a Blue Sea junction block behind the dash. It's fused on both sides of the firewall and has been nice for ad ons.
One thing to consider is the interference that a lot of the high power LED's cause. Currently only my center dome light causes any interference when using my HAM radio. It's the driver that causes it and I haven't really done anything to fix it. It's just white noise. My Blue Sea dual USB port on the other hand was far worse. It's mounted near the Blue Sea junction and the radio. It took some jangling to fix that.
Now I've got the same interference with the portable battery box i made recently. I have a noise filter I may try but first I need to order some ferrite chokes. My inverter and USB ports cause the same noise. The radio is attached directly to the radio and it's still an issue.