The reverse gear in MR2s is very basic: the idler gear, responsible for shifting into reverse, it is straight-cut and lacks a synchronizer, unlike the other forward gears. Special care needs to take place when utilizing the reverse gear since you, the driver, will be playing the role of a synchronizer!
On occasion, the teeth on the idler gear and reverse gear jam, preventing a fluid shift. To shift into reverse, the idler gear needs to be rotated to mesh correctly. One way to do this is to shift into another gear, then back into reverse. Also, if you are precise, you can hold the shifter into its reverse position with light pressure and very gently let out the clutch to roll the idler into synchronization and shifting into gear.
Grinding occurs when the gear teeth clash when rotating, whether the car is moving or not! To correctly shift, push the clutch in, wait a moment for the shaft to stop, then shift into reverse. You can also stop the shaft quicker by shifting into another gear and using its synchronizer as a friction source.