All of the shift interlock parts are inside the gearbox, some buried deeper than others.

-- Start with remove the tunnel center carpet to expose the tunnel cover.
-- Remove shift knob and rubber draft excluder boot.
-- Remove eight #10 screws to lift off the tunnel top cover (except for the first 500 cars from production).
-- Remove four bolts from front of the opening, and lift off the remote shifter assembly.
All of that might take about 20 minutes, if you don't have too much trouble with the carpet.

A small bronze split bushing with wire retaining ring should remain attached to the front lever of the remote shift assembly. A flat plate with two dowel pins should remain attached to the gearbox rear housing (but may come off with the shift extension assembly).
-- Grab a 3/8" drive socket wrench extension, about 8" long. Hold it big end down, and insert it into the hole in the remaining shift lever. Use that as temporary shift handle.

Without the remote shift assembly, the shift pattern with be re-oriented 180 degrees, so
1st will be to right and back,
2nd to right and forward,
3rd to left and back,
4th to left and forward,
reverse to far right and forward.

If it shifts properly (clutch depressed), you can drive the car that way. It it doesn't shift properly, you can apply a lot of force there for inspection purposes. It should require about 3X as much force as the normal shift lever top knob. See what moves and what doesn't move.

If anything is still not properly functional there, then you will need to remove engine and gearbox to dig deeper. If you are a glutton for punishment, you might remove the floorboards and tunnel cover for access to gearbox top and side covers. If you did remove the tunnel cover, you might be able to remove the gearbox without moving the engine.

With gearbox exposed, remove the top cover from rear housing for access to the shift selector interlock arm. If the top cover was installed backward (as illustrated in factory literature), it might prevent shifting into reverse gear.

Lift out interlock arm, and pivot the selector shaft front arm out of the gates. You might need to remove the square plate from top of rear housing (see top of page) to pivot the shaft far enough to clear the gates. See CSM MG269 for fixing a faulty interlock arm.

With the rotating control lever out of the way, you may push/pull the three individual shift rods fore/aft.
-- Three selector gates all aligned would be neutral position.
-- One shift rod controls 1st and 2nd gears.
-- One shift rod controls 3rd and 4th gears.
-- One shift rod controls reverse gear only.
-- With any one gear engaged, the other shift rods should not move (locked out).

It should not be possible to move two rods out of neutral position at the same time.
Interlocks between the three rods are in a small block on rear end of the main housing.
.The rear housing must be removed for access to that interlock block.
Those rod interlocks are seldom any problem.

-- If a shift rod moves without engaging gear, then most likely the bolt securing the shift fork to the rod is loose.
-- If any gear is engaged when three rods are in neutral position (gates aligned), , then most likely a bolt securing a shift fork to the rod is loose.
-- You need to remove the side cover for access to the shift forks and the fork-to-rod securing bolts.

Removal of the side cover gives access to the locking bolts securing the three shift forks to their shift rods. If you did remove the tunnel, you may have access to the top and side covers while the gearbox is still in place.
The fork to rod locking bolts are often a problem if they were not properly seated and jam nut locked during assembly. A pointed tip on a locking bolt needs to seat in a drilled hole in the shift rod. Then snug up the locking bolt, and ighten the jam nut to assure the bolt cannot come loose.