There are two main sorts Wreford.....wired and transmitting.
The wired ones, though a bit more faff, are hugely better..and sod the £120 fitted..it's a piece of pish to do yourself
You take the head unit out (two bits of coathanger if you don't have the forks to do it, or Halfords sell the forks for around a fiver) then find a 12v supply in there.......you can even use the switched supply from the ignition to the head unit to tap into.....that supply goes to the modulator unit (which you'll also need to earth

) then you simply unplug the existing aerial, stick it into the modulator then plug the lead from the modulator back into the head unit aerial socket.....there will also be a jack plug flying lead which you can run somehow (grommet or small hole or even from out of an air vent or something) that you plug into your ipod or whatever, into the headphone output socket.
Effectively, you've put in an inline input to directly to the aerial feed.
You then tune the radio to one of the given frequencies and voila!
The other sort id just a low powered FM transmitter that uses (usually) a ciggy socket for power, then transmits the signal you're putting into it which is picked up by the aerial as if it were a normal radio station broadcasting.
The difference in quality is obvious.....one is hardwired fed into the unit and the other is a weak signal picked up by the unit.
With the transmitted type, you'll get interference (if someone next to you in the traffic queue is using the same frequency, you could end up listening to their shyte hip hop

) and it'll never be as good as the wired version.
The only thing the transmitter type has going for it is that it's portable to any vehicle with an FM radio and a ciggy socket.
HTH
It's certainly not worth paying to get it installed.....as long as you can get the head unit out and are capable of finding a 12v switched live and an earthing point, which obviously you are