The antenna on my 88 used to work fine and now stays in the up position.  It still makes the same noise when you turn the radio off like it's trying to work in fact a couple of times if you slightly push down on the top of the antenna, it would go down but no more.  My question is do I need to buy the complete assembly or some other easier part?

could be the strap inside is torn. i have seen replacement strap for other vehicles and maybe there is one for yours. i would also check if any other gm cars use the same antenna. on my 75 i bought an electric antenna the was for a caddy for like 60.00. it was the same exact antenna that was being sold for the corvette at about 150.00. the only thing that had to be done different was you had to add a ground wire to the mount as corvettes are fiberglass.