Originally Posted by HillRider

Remove the seatpost and see if the top tube is open at the seat tube end. You may be able to stick a finger into the top tube and either find the obstruction or guide the cable into the exit ramp. My '92 Trek 1420 had internal rear brake routing and no guide channel. Guiding the cable out the exit hole from inside the top tube was the olny way to do it the time I inadvertently removed the cable and housing together.