Depending on the radio, you may be able to release it out the front using a U-shaped hunk of coat hanger wire, or the actual tool if its around, to release it out the front. (pix around here somewhere of the process)
If its that type, you'll find two holes on both sides of the front that are rougly 1/8" in diameter. (they may be under a removable faceplate)
Tool goes in, presses release latches, radio pulls out, wires disconnect once it's pulled out enough.
The gauges can be gotten at by removing the glovebox and going in through the hole it leaves behind.
Remove door by taking out the screws along the hinge, light, door stop, latch come out and any other ecrews that might be lurking in the box proper.
Carefully weasle the box around til you can get it out of the hole.
(its plastic, but odds are more in favor of cracking the dash by wedging it out of there than breaking the box itself. A broken glovebox can be hidden by closing the door, a broken dash screams 'hey, look at me!!' and can be costly to deal with)
Once out, you just reach through the hole to the backside of the gauge area where said gauges are held in place like any other conventional or aftermarket gauge: couple nuts holding a yoke (they will have washers too, so don't drop them unless you've got replacements) the yoke proper, light and service wire, service wire(s) for gauge (held onto the gauge with more nuts)
If the radio doesn't have the conventional release as above, or is anchored to the chassis by the security strap, it too can most likely be accessed once the glovebox is out of the way.
Or, you can strip the console and side panels if the radio is the type that has to come out the back.
