I used fibreglass rods to run along under the roof lining and grab the cable. Working from front to back seemed easiest If you take a line just about under the roof rail that's about right, too far over and you can't get past. Pull the lining at the back down to find the rod (took me about 20 mins working back and forth) once you have both ends, attach your cable and pull through. Needs to be pretty strong and stiff rods to stop it curling up, a plastic snake will not work.
The next bit was both easier and harder than expected. Pull the rubber boot out at both ends, it's fair stuffed with wires so be careful. The car body seems to be a double skin there so be patient passing your cable through the bottom hole.
I used a long thin zip tie to help feed a string through the boot and the taped the plug to that. Greased the outer tape and it pulled through easily from the bottom up.
Next difficulty was passing a rod from the centre hole out through the cable entry, it's a tight corner but once done all you have to do is feed the wire along and out where you want it and tidy up. I tucked all the spare wire in under the roof liner.
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