91 passat gl stuck in 3rd

I recently bought a 1991 passat gl, 2.0l 16v 5spd, in perfect condition except that it is stuck in third. The gentleman i bought it from said he was on the freeway and he downshifted on accident and now it wont shift from inside the car. He told me that you can go underneath the hood and change the gears but when you try and shift from inside the car that it wont shift. My real question is what is that part? and how hard/expenisve is it to replace?