Well, there aren't supposed to be little "slots" on either side of the dial. Did someone (PO) break off the tabs? A light push on the tab is supposed to release the clip and then the tab will come out so that you can pull the plastic pieces and the unit out.
My much abused 780 had that problem, and I finally figured out that just using a homemade paperclip with a 180degree bend at the tip worked best to pull the tabs out. On the upper/back of the unit is a plastic clib which is supposed to retract to allow the radio to slide out. I don't think it is reachable, but you might try to find a piece of thin plastic to insert from the back side to push the plastic clips down, so you can slide the unit out.
Sometimes, a slight push/pull on the unit is needed to help the rear tabs go down.
Hope this helps,
Klaus
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(1975 164, 1995 854T, 1998 V70R)