... this is a one off application?
You know the frequency and you have a car - I'd construct a hand held or pole mounted 2 or 3 element Yagi for 173.075 Mhz, set your receiver for 173.075 (select as tight a bandwidth as you can if your receiver will let you select bandwidths - the narrower the bandwidth the greater the receiver sensitivity), connect up your DIY Yagi and rotate it for best signal strength, draw a line on a map, head off in that direction, jump out the car, repeat the above (i.e. rotate the antenna), take another max signal strength bearing (note it down on a map and draw a line).
Within a couple hours, so long as the transmitter is not hundreds of miles away, you should be able to get within a couple hundred yards of it - then look for the antenna.
To quickly elminate the possibility of the antenna been hundreds of miles away, take your first bearing and drive off at 90degrees to the the first bearing for a few minutes and then take another bearing with your antenna. The larger the angle where the 2 bearings intersect the closer the transmitter - and to be extra sure drive off again at roughly 45degrees to the second bearing for another few minutes and take another bearing. If all the bearings cross-over at roughly the same point then just head off to the area where they intersect.
Sounds basic - almost stupid, but you'll be suprized how well this can work, and how quickly you can narrow the transmitter location down to a few hundred square yards. Even if your initial bearings are as much as 15 - 20 degrees off, giving you an initial intersected area of several square miles, as you get closer and take more bearings that 15 - 20 degree error (and you can be sure there is going to be bearing error in the region of 20degrees or so, if not more), it will quickly start to represent less and less surface area.
