Terence,
You do have to access the back of the tach, which is where the bulb housing is at. I've found the best way is to remove the drivers seat first. Sounds like a pain but it will save you a rotator cuff if you take it out first. Then laying on your back with you head on the brake pedal, work your hand into the back cavity of the dash. The tach is held on by a knurled nut (brass I believe) and a "U" shaped keeper. Remove the nut and the tach will slide out the front of the dash. You will have easy access to the bulb and can easily replace them. There is a ground wire attached to the keeper by the knurled nut, you want to make sure it is in place as you put things back together again.
The whole procedure can be done with seat in IF, you are very flexible and IF you have small hands...or if you've done it 10 times.
I just wish they made those bulbs brighter!!
Good Luck
Tim Dunagan
tdun824259
1971 280 SL