There is a mod available I think from poorboys racing or some shit, about using a dremel to make an accessory door just in case the repair is ever needed again. U would only need to drop.the glovebox and pull open the access door and remove the heater core. Just throwing it out there.
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Ryan
1990 Mustang GT (In Process)
1993 LX Coupe (Daily Driver, but just choose not to)
2005 Excursion (Big Pig)