Either DIY, or find a good independent car audio specialist. I had a head unit fitted by Halfords, it took the bloke nearly three hours to install the unit, a fairly fancy Alpine, it involved getting the head-unit to work with my Genesis amp, which was never quite right afterwards, and when I bought a DAB unit, (from Halfords), I found a fairly local outfit who did the install in around an hour, including fixing the bodged wiring that the Halfords bloke had done.
Still using the little stick-on windscreen antenna, but I’m getting them to fit a proper antenna soon; no way will I trust Halfords to do the job, I’ve been into the shop three times to find out what they’d charge, and waited around for fifteen-twenty minutes without managing to find someone to actually talk to!
Really, avoid at all costs.