On my '99 SR5 manual, you need to pull off the whole center trim panel pulling from the bottom. You have to get the bottom clear of the center console by a little prying. FSM says to actually remove the center console around the shifter first. Once the trim is off, the HU is held in by four screws into the sides of the HU.
I suppose someone could have used an Alpine sleeve where the HU slides out, but it would be more work and a poorer result in terms of solidity. If the sleeve is there, you can see it with the face plate off. Newer Alpines come out with two flat metal tools down each side like a narrow butter spreader, the older ones used the two holes on each side type--the right nails or drill bits will do it. (Lots of Alpines in my life, I have bought nothing else since 1980.) But you probably don't have a slide-out setup.
I agree with Stick--Dual made great turntables in the seventies but their present day electronics are pretty bad.