Aftermarket Tach

Hey, i was wondering how hard it would be to put in an aftermarket tach in a '98 Lesabre, and if anyone had done it , if so any help would be appreciated, thanks

Oh and also another thing...my courtesy lights are going out one by one slowly...my right door is dark, my heater backlight is out..except for the lil orange light, the "twilight sentenial" switch, have gone out now, is this just aging or is sumthin wrong in the wire ing? thanks