Most of my slide guitar pickup installations use a 'barrel jack' glued into the tailblock. The system is effective and looks good, but if you want to change the pickup or the modify the wiring you will have to replace the socket. This is not a problem as the socket is cheap and the process is simple.

Here is step by step instructions on how to replace this part.
Even if you take the guitar to a professional repairer, you should refer him to this page so that he knows how it was installed.

This is a fitting sometimes called a barrel jack. They are used in a lot of modern electric guitars and basses, and should be easily available from a music shop. I use them on my slide guitars because they look good and they don't stick out like an endpin socket. Take the nut off. as it is not used in this installation.
Set up the guitar on a work area where you can push the guitar head against the wall. Cushion it with a bit of foam or something soft to protect the finish.
For a floating bridge guitar, loosen the strings and rotate the bridge out from under the them.
Now remove the saddle and pull the pickup out about 2 inches. (You can get the pickup out by pushing the lead up from under the bridge.)
For a fixed bridge guitar, start by taping the strings securely over the nut. Then loosen the strings, remove the bridge pins and carefully pull the ball end of the strings out of the bridge.
Now, using a small bit of wood etc. carefully lift the pickup out of the slot from the treble end. Take care to lift the pickup horizontally so that it doesn't stress the solder join on the bass end.
Cut a slot across the end of the socket with a hacksaw while holding the blade in position with your fingers.

Do not remove your fingers until you are done cutting!
Cut the slot about 2 mm deep. There is a rubber washer behind the head, so you can safely tap it with a mallet to loosen the glue around it.
Using a wide flat screwdriver, push against the wall and unscrew the socket a couple of turns. (anticlockwise) It should move easily after tapping the socket with a mallet.

Always hold the end of the screwdriver in position with your fingers!

After turning the socket with the screwdriver you should grab the lead of the pickup and turn it clockwise the same number of turns.
Continue turning the screwdriver and pickup until the socket head is at least 12mm(1/2in) out from the guitar.

Now, if you want to speed things up, you can attach some vice grips to the head of the socket and turn it at the same time as rotating the pickup.
When the socket comes out you can put the pickup, saddle and bridge back under the strings.and cut off the old socket.
Push the lead back inside the guitar and clean up the hole with a round file so that the new socket slides in snugly. (not loose)
When that's done pull the lead back out with some bent wire.
Before connecting the pickup lead, it is a good idea to solder the earth terminal to the longer pin, which is the ring or middle contact. This tends to prevent noises during performance and effectively makes the socket last longer by increasing contact. Solder the center wire of the pickup to the short pin and the outer wire to the ones you joined.

After wiping a small amount of oil or grease on the socket threads apply some wood glue to the inside of the hole. You can also put some glue on the threads but be careful to clean off the excess as you push the socket into the guitar.
(If you have accidently made the hole too big, you can wrap a little paper and glue around the thread.)

Putting some masking or gaff tape over the plug overnight will stop any movement and make sure that the glue sets strong.

Position the bridge so that the front edge of the saddle on the treble side is 645mm from the nut. (1 mm longer than the actual scale length) The distance should be about 646.5mm on the bass side. (1.5mm longer than scale length)

treble = 25 3/8 in
bass = 25 1/2 in