Ric truss rods work by having a threaded end with a nut that bears down upon the aluminium block you can see near the nut. The rod then travels down the neck and at it's furthest point from the nut is folded back on itself and is bought back towards the nut until the 'free' end meets the back of the same aluminium block that the threaded side bears on.
Basically, as you tighten the rod, the whole thing gets pulled tighter through the 'block' holes, the free end that is jammed behind the block can't move anywhere so the free half of the rod can't do anything but bow upwards exerting a back bow pressure on the neck.
Harder to describe than visualise when you see the rod in action.
What it means, however, is that the rods are not glued into the truss rod channel in any way, so if you take the strings off, get a thin walled 1/4" nut driver and remove the nut from the other rod, you will be able to remove the aluminium block and the truss rods can be slid out. You may be lucky to get enough of a grip on the broken rod to tease it out of the channel.
Once that's done you just need to slide a replacement rod in. Replacements are available from Rickenbacker, but given their sporadic stock levels, it's not rocket science so it should be a simple enough job for any engineering shop to make a duplicate rod using your good rod as a pattern.
When you're setting it up again, remember that on a Rick 4001 the way to set the relief is to manually bend the neck to where you want it and then just snug the nuts up. The truss rods are only designed to hold the neck in place rather than shift it. Trying to shift the neck by overtightening the rod is probably how the rod got broken in the first place.
This
link might help
