97 406 GLX 2.0 Petrol - Replacing the Modulator Box

Can anyone please tell me how I get at the Speedometer control box (Part
number 1929SJ) also I think known as the "Modulator Box".

I know it's somewhere up behind the glove box, but despite trying I can't
get the glove box out to find the little devil. Any help gratefully
received.

I've checked with Peugeot and my car is fitted with one.

.......................................Symptoms of the fault which make me
think it's this box.........................

The Speedo works well until I've driven about 150 miles when it starts to
fail intermittently. As soon as this happens the engine refuses to idle or
run smoothly. When idling, the revs drop very suddenly to 0 and then
suddenly back to 1000 sometimes it stalls. After a few days all returns to
normal until I do another 150 miles and then it all begins again.

Thanks
Chris