I haven't done it, but I would run it right up the A-pillar and up under the headliner to where you want to put in a pass-through. Access to the area between the glovebox and the A-pillar is under the cup holder, which pries off easily. The A-pillar trim also comes off pretty easily by prying up the edges and firmly pulling, but there is a leash on it because there's an airbag behind it. The leash takes some wrangling to twist and pull in two different directions to get off, but you can likely just leave it in place. But while you're in there, you might as well put in some good tweeters and run a new wire to your door--faroutride.com has some good info on this, as well as
@Hein Youtube channel, DIYVan. You don't have a signature indicating what type of Transit you have, but if you have a cargo, the cab headliner drops easily from the back by prying the three retainer clips near the center giving room to reach in and get access. If you want to run 10 or 12 AWG wire, a solar gland like this would work for a pass-through:
3/4" NPT Solar Masthead | M3234GBS-SM | ElecDirect. Alternately, something like this for 10-12 AWG, also used for solar:
Amazon.com: Customer reviews: Link Solar Weatherproof ABS Solar Double Cable Entry Gland for All Cable Types 2mm² to 6mm² for Solar Project on Rv, Campervan, Boat. For smaller wire, I'm sure you can source something, as well.
I suggest getting a trim panel removal kit, if you don't already own one, which will help keep from breaking clips and trim:
Amazon.com: 19Pcs Trim Removal Tool,Car Panel Door Audio Trim Removal Tool Kit, Auto Clip Pliers Fastener Remover Pry Tool Set with Storage Bag: Automotive