The radio comes out from behind the dash if original 2-knob style.
The A/C ductwork and lower plenum may need to get loosened or removed for more room. Pulling off the lower valiance (chin) may gain you some more wiggle room to get under there.
On mine with a bench seat, I put the seat all the way back and bent myself into a pretzel for a while. I just considered it my free yoga lesson...
If you have a console and buckets, you might have more advanced yoga to do...
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-Rob Young
1972 Cutlass Supreme Convertible (442 clone) -"Lady" (My mistress...)
http://www.flickr.com/photos/robsalbum/sets/
1986 Cutlass Supreme Coupe - "Pristine"
1978 Ford (Old Faithful) a.k.a. "the Tramp"
1997 Cadillac STS (for sale - too many cars!)
1999 Harley Davidson Sportster - "the Freedom Machine"