if your new wood trim piece is attached to a metal backing then all you have to do is press it into the plastic plate that the acc is 'set' into. you will notice 'bumps' in the back of the metal trim plate that matches to the plastic acc plate. if no metal backing then take the old one (if u got it) and glue the wood trim onto it then try the above. if u are completely missing the metal backing to the wood trim then search this site / make a request in the parts section. if you have a stock radio it helps to take the face plate off before replacing the wood trim and/or remove the ashtray for better access.
good luck . btw, how many miles on your 84?