4-wheel drive vacuum actuator tube?

One of the tubes to activate the four-wheel-drive broke a number of years ago and four-wheel-drive stopped working. The tube was patched and repaired, and it worked fine for a while. Now it has stopped working again and I am assuming the tube has broken again but I can’t seem to find it, any pointers on where it is? I thought it was on the passenger side near the battery, but I can’t seem to locate it. Your help would be much appreciated!