Hi JJ, I posted this on the other thread but thought I would post here as well for anyone searching for M4 interference. The M4 is the main culprit, PC can be generate some but to a lesser degree and finally any device with an SMPS, such as a (some) cameras. It's a pain to have to go the expense and hassle of fitting the PSU into a correctly RF screened case, and even then placement and lead dressing around the PC are still important.
It's not going to be easy to find a PSU that is (truly) free from radiated interference. You really need to find four or more people with PSU brand X that are running DAB-FM and GPS and have tested reception using a laptop (PSU on and off) out of the metropolitan area where reception is not saturated with DAB-FM signal levels. I have seen people fluke an installation with an M4 and have no reception issues in their location. I have technically proved beyond any doubt that the M4 is a noise generator, but so too are most SMP supplies, it just comes down to how much they produce.
As others said, an external roof mounted aerial helps as it did in mine, but won't solve the problem when signal levels start to drop.
Like I mentioned before, a tiny 1/4" x 1/4" SMPS in a camera completely killed DAB in my vehicle and no amount of simple ferrite beads, shielding or lead layout would fix it.