I've got a Gentex mirror on my '96 ES (same mount). The factory mirror is held on by some very stubborn adhesive, and there's a rubber damper between the glass and the metal mount. It took me a dozen glass shops to find one willing to do the job (most howled that the windshield would break). Finally, I found a shop willing to do it. The tech there had seen the Lexus mirrors before, and knew that the trick was to attack the rubber vibration damper. He took a putty knife and ball peen hammer, and started tapping away, while another tech pushed down on the windshield from the outside to keep it from flexing to the breaking point. They got it off in about 30 seconds, and the tech cleaned the residue off with solvent, and mounted the American-style mirror button for me. Didn't charge me a cent :) I slipped the tech a crisp $20, and told him that he had absolutely made my day for helping me pull of a mod they said couldn't be done :)
Once I had the mirror mounted, it was time to wire. Pulled off the A-pillar trim (snaps off, starts above driver's window, goes down into dash. Slipped the wire behind the edge of the headliner (remove the screws for the driver's side sun visor and center sun visor to pull the headliner back). Removed a piece of weatherstripping at the bottom of the A-pillar, to run the wires down into the dash. Pulled lower dash panel out (4 bolts, two of which were hidden). Run the wires into the fuse panel to something switched with ignition, such as the cig lighter) using fuse taps. Secure the wires to the A-pillar wire bundle with zip ties. Replace headliner, A-pillar trim, and weatherstripping.
My mirror was a bit more complicated to wire, as it had a separate power feed for the dimmer and compass (switched with ignition) and the HomeLink garage opener (always on). I had a stereo shop wire the "always on" portion for me. Unfortunately, they didn't replace that weatherstrip by the dash, and I had to buy a new one for $23 at the dealer.