It would be helpful to detail what you've done, what equipment is there, etc..

Generally I try to modify the wiring to be as close as possible to John DeRosa's excellent presentations. I haven't found much on the topic of radio interference on his or any other website but would be happy to be corrected.

I've checked for voltage drops to the instruments, cleaned and tightened terminal blocks, grounding, fuses and connectors, separated Radio wiring as best I can (both physically and electrically), placed split ferrites on power, spr and mic/ptt leads to radio. Looped (twice) power wiring to the flarm through ferrite rings, clipped split ferrites on data leads to FLARM display and card readers if present.
I find that the first minute after the Master switch has been turned on is the noisiest, I assume things are initializing during this period. The Flarms seem to be the "primary culprits". A handheld radio confirms this.
The noise is stronger on some frequencies than others, 122.7 Mhz is moderately quieter than frequencies above 130 Mhz.
Should I try a ferrite on the Flarm GPS antenna or will this mess up the GPS?
Also, is a ferrite on the radio antenna cable a really, really bad idea?

I have assumed it is better to run both positive and ground wires to an instrument through the same ferrite OR would it be better to have a ferrite for each wire? Would it be better to have no ferrite on the radio ground so that its chassis is 'closer to RF ground'. Sigh.
I haven't tried a "12v interference suppression capacitor" yet, will order some. Will also order some copper foil to wrap around the flarms (or possibly to make a hat