I have removed the dash on two 300ZX cars to replace the heater core valve - which typically fails due to having the engine cooling system overheat after a timing belt replacement - where there was a failure by the service person to remove trapped air from the engine cooling system resulting in superheated air blowing the hoses off the cooling system. It is a two day job [noon to 5pm with breaks and 9am to noon with breaks] for a mechanically inclined non prfessional mechanic. The job is time consuming but can be done.
Twinturbo. Net may have some posts where some say they can replace the core by removing it from the drivers side of the transmission tunnel in the car. I would be afraid to try this as it involves laying on your back and making some Hoodini hand neck and arm movements.
The reason it takes two days, with the dash removal, is that you have to take your time removing the dash piece by piece to avoid breaking plastic and to ensure everything is operational before putting it back together. If you try it get the service manual and take your time and make sure all the screws and bolts go back where they belong. I am no expert and was able to complete the repair like new, twice.
Best of luck.
Thomas E. Shafovaloff
Winter Park, Florida
Tuesday, February 12th, 2008 AT 4:06 PM