How long do the batteries in the key remote really last? My 99 Eldorado has 70k miles, and I can only open the car from a few yards away. Often I have to stand there and try several times, which I annoys me given that everyone else on the parking lot seems to be able to open their cars from 30 yards away with the first try.

Is this supposed to be this way? I've always had to stand relatively close to the Eldo since I bought the car used to get it open. Also, I've always had to hold it in such a way as to point directly at the car, my other car opens from really far away, and I don't even need to hold it in a direct line to the car. Strangely, the trunk opens from farther away than the car itself, how is that possible?

If the battery in the key remote is depleted, how expensive is it to get it replaced, and can they damage it by opening it?

the signal strenght on the Eldo always sucked

I always heard a strange story that a lot of people swore by. If you hold the remote up by your mouth, something about your fillings boosts the signal.
The guys at cadillacforums swear it's true.

Well, that wouldn't work for me. I have only 2 fillings, one gold and one ceramic. I brush regularly!

I read in a different forum that the antenna for the remote is somewhere underneath the back seat, and that all you have to do is lengthen it with some wire to give more range. I'm not sure yet whether I really want to tamper with that.

It's not under it, it's mounted to a panel on the back of it. That's where the control units for a lot of the electronics are. ABS brakes, Bose stereo, Keyless entry etc. You get to it by removing the cover in the trunk.