I suspect I have seen the answer - the brown wiring mod! but can someone please comment.

My symptoms are my radio stops when my wipers are on intermittent, especially at idle. I also note that my horn doesn't work when the engine is running but works fine with the engine off in the garage.

Cheers

Actually no, it sounds as if you do have some sort of electrical problem other than the usual low current caused by overloaded cables/ high contact resistance.
Have you checked the voltage output of the alternator? Should be 13V - 14V above idle, and that should be enough for the horns.
Related item, are your horns (both of them) operational and tuned? Out of tune horns may not sound at all and draw lots of current. Careful when tuning horns, they are VERY loud!
There may be some mechanism whereby at a higher voltage the membrane of the horn doesn't vibrate, but that would imply a considerable voltage difference between engine on and off.
The radio is puzzling, but it could be down to the wiper relay drawing loads of current in intermittent mode.

Thanks for that. I did do the brown wire mod at the weekend and it certainly seems to have addresed the issues. I am interested in the comment about tuning the horns. I haven't seen anything about this or how to do it. Can you provide any details.

They're hidden under the headlamps, you can access them through the hatch. I seem to recall there is only one screw holding them (one horn each side). If you remove one you'll see there are three screws (?) holding it together, and them there is one screw that seems oddly placed. That is the screw that controls the pitch of the horn. I wired my horn directly to the battery to tune it. Stupid idea since a horn at close range is really loud. I believe there is one high and one low pitch horn. I only tuned the one that was not working.