Theres usually a rubber gromit that u can remove or cut through to push it through on the passenger side bulkhead...if you have the passenger airbag it can perhaps be a little bit of a game but stil not impossible.
What model car do you have...and engine size....8v mk3 and to a degree 16v are fairly simple as there is room to play with..the VR6 engines can be a lil bit more trickey but again..not impossible..jus takes a lil more time n patience realy!few threads on here somewhere that states where others have routed theres so mite b worth lookin them up in this audio section!
