No risk whatsoever.

If you were trying to supply device that might demand a heavy current you could overload a circuit by tapping in the wrong place (the wires supplying where you tapped into might be too small.)

However the antenna you are supplying will draw a few milliamperes (as in thousandths of an amp) and there’s no problem.

Thanks...really appreciate everyone taking the trouble to reply.

Just to add... successfully installed new Nakatanenga aerial over the weekend. Bit of a fiddle getting the new lead through the dash to the stereo but other than that all pretty straightforward. And what a difference! Really worth the effort. The reception is SO MUCH better than the original aerial and gives the option of DAB as well as FM. Also the aerial itself looks much more rugged and is half the size of the original - much less likely to be damaged by overhanging branches when motoring down narrow lanes. DAB performance is really good although you might need to buy a SMA to SMB connector separately to get DAB working - I did. This aerial wasn't the cheapest but well worth paying extra for in my view. Could not be happier with it! (fast delivery from www.4x4overlander.com too.)