Richard, Unfortunately the van is fitted with a satellite dome so no aerial to tap intoPaul, I seem to recall the aerial is located in the driver's side bus type mirror. If you look up between the bed and the front sliding window you will see a junction box screwed to the inner wall. Remove the cover and you should see the aerial coax lead. Not sure how to get to the aerial in the mirror though asI Iwired my head unit directly to the Status 570 TV antenna booster box (radio f male terminal) and this gave a better signal for FM and also good reception for DAB.
Have you also checked that the aerial lead is fully located in the head unit? You need to pull out the head unit to check this but it's a relatively easy job as you should have the removal keys as they are supplied with the G749C manual for the head unit.
Again, hope this helps.