I changed my ugly aerial for a shorter one and the radio signal has suffered. I know I could change it back but I hate the way it looks. I don't want to change the whole aerial and have to rip the headlining out.

Headlining isn't fixed at such. It's kinda help in place by the edges.

 

When I ran in my rear DVR I took the front sunvisor and front handle off, released the trim at the back and used a piece of long, flat, plastic, to pull the cables through. Far less of an ar$e-ache than you think, and very quick.

 

IIRC the aerial is amplified anyway. If you've just replaced the mast, it could be that it's not really designed for the job. There was a couple of Ebay for around £60 that are "all-in" (so FM, AM, DAB, Phone) that are quite small (shark-fin and stubby aerial.)

http://www.ebay.co.uk/itm/151680171966

An amplifier might help a bit, but not a lot. They can compensate a bit for low signal but they also amplify any noise and add some of their own. So the relationship between signal and nose doesn't go up that much. If you are in a good signal area it might help, but in a poor area there just isn't enough signal picked up by the shorter rod so it still gets lost in the noise even when amplified.