Well, I guess you can use a magmount and run the cable thru the roof side of the door opening. I assume that X-Cab Tacomas don't have a door for the 2nd row of seats, if that's not the case, you can run it thru one of those doors, or, mount it on one of those doors.
Most contractor vehicles down here have a steel thing behind the window, you can see it here http://www.flickr.com/photos/43033468@N00/2460740514/
, if you have something like that you can mount the antenna there and route the cable the way I told you.
Another option is (sacrificing your AM/FM radio), you remove the antenna and see if you can route the cable thru there, I know they did that in some police cars here, they ran the cables for the lightbar/speaker thru there, but they completely removed the AM/FM antenna.