You said, "getting in there." Can I assume that the dirt is inside the lense rather than outisde?

That depends on the construction of the lens package and how it is attached to the vehicle. Usually a lens unit is a number of pieces of plastic that are seperately molded and then heat bonded together. The goal being to create a single piece which can be sealed from the weather, dirt, etc. Now, in general the only openings to the sealed lens unit is through the light bulb hole. The lense unit is usually attached to the vehicle using some kind of screw or removeable rivite system which firmly attaches the lens to the vehicle and the holes for light bulbs extendes into the interior of the car in the trunk area; so that end users can get to the light bulbs for DYI service. Sometimes there is an o-ring seal at the base of the light bulb holes which presses against the body of the vehicle so that water and dirt cannot enter the vehicle either.

Now, a lens can get dirty in a couple of ways. It can loose its seal on one of the heat bonded seams, or dirt can enter through the light bulb hole, although that is pretty unlikley because the light bulb should keep dirt from entering and the light bulb hole is exposed to the interior of the vehicle, in most instances.

However, if you can figure out how to remove the lense from the vehicle you can actually wash out the lens. A light soap and water solution swashed around inside the lens might clean up the dirt. Rinse very well afterward and let dry until totally dry. If the lense leaks water from the inside you have a seam which is not bonded properly and as such the lense will always be prone to getting dirty on the inside. Might be time to think about replaceing the lens then. You can use a soft brush to "scrub" the inside of the lense, just don't rub hard or use anything that scours or has a harsh surface. Think terry cloth towel in texture here. A soft bottle brush might be the answer.

If inside dirt isn't gone after this, again, might be time to think about a new lens

It has been said:
Hybrid drivers come in 3 flavors, greenie, techie and cheapie. Pick any 2.
2005 Prius, Melinium Silver over gray, package 5 (AI)