Best way through firewall

[XTheOwl]

I gutted a radar detector and I am gonna try to make it in dash. I need to run some cables from the engine compartment to the dashboard. Anyone know a good way to get through the firewall?
I know on my bud's jeep, there was a little rubber stopper you could pull out of the firewall, just in case you ever wanted to put on foglights or somthing. Theres nothing like this on the mustang is there?

Thanks,

xto