Defroster intake access firewall cable/housing

[Robert E Fox]

I have a 63/250 SN 3550 with a Defroster that I can't completely close the valve to prevent the heat from coming into the cabin. The scissor like or butterfly valve partially closes when we look at it from outside the firewall with fiberoptic light but does not completely close shut. You have four firewall nuts easy to get to but inside the screw [bolt] heads are not easy to get
or better yet impossible to get at. We want to change out the wire and housing any suggestions other than taking out lots of avionics and wiring. We could cut the four nuts and see if the unit would just come forward in the dash then maybe rivet the unit back on? We lubricated the cable and housing this did not work.

Looking for suggestions tired of using Tin-foil to block the intake tube.

Thanks, Bob