I recently acquired an abandoned 1975 X19 and cannot get the rear trunk open. I was able to get the engine hatch open by getting under the car to release the lock but the cable that goes to the rear trunk was disconnected. There are no release levers or cables in the body cavity behind the drivers door.
Any ideas???

Also this car is a 1975 but has a 5 speed?? Is there any way to tell a 1500 from a 1300?

It is a bit of a nightmare situation!

Fiat did think of it though and there should be an emergency cable in the engine bay (and one in the trunk for opening the engine cover). It is basically a thin wire wire a plastic ring handle and should be tucked away at the back of the engine bay on the driver's side (for you). The only problem is that the cable is pretty feeble so they tend to snap.

The alternative is to cut the bottom of the boot out (above the exhaust silencer). This is a skin of metal forming part of the chassis. A thick layer of insulating mat and then a screw in panel on top. Normally the bottom panel rots out anyway so you should have little trouble there

There is no immediately obvious way of spotting the difference between a 1300 and a 1500. The 1500 is slightly taller but the real tell-tale is the engine block casting number. The 1300 starts 128AS while the 1500 starts 138AS.

If you still have the earlier "flat" engine cover and the OEM air filter then it is definitely a 1300, the 1500 doesn't fit, hence the later engine cover.