I suppose you could try to split open the lighter plug to see if there's a faulty connection inside it. The early GTM 25 and charger-only cables with the plug end as shown in Loly's pic earlier have a screw under the label and can be easily opened for repairs. The later Garmin receivers/chargers like yours i've never opened. The one i mentioned earlier with the fusable link i've now found on close examination to be a non-genuine generic charger lead. It came with my first [bricked] 3790. I split that plug open, but busted the plastic lugs at the cord end. It was easy enough to then bypass the fusable link using smaller AWG wiring patched over it as a 'jury-rig'. Problem was it's had to be taped together. You may have the same problem getting the genuine plug apart without damaging it i think. Maybe someone has opened one and can give you some tips. You can buy GTM25's on eBay for around $20 from China usually they have US/CA subscriptions. They work fine with 37x0 series and with gtm25 there's no problem adding a JM sub for Brazil: [Only registered and activated users can see links. ]. That might be your cheapest and easiest way out.