1990 mazda mpv

The Hold light is flashing because of an unknown failure. You will need a scan tool and diagnostic skills to repair. A common problem is a broken wire below the shift lever housing. Please let me know if there is anything else. Good Luck.

that reverse light switch just threads in no adjustments

a simple contact switch

does reverse gear select firmly no slop

are the cable connections to the gearbox to the shift stick sloppy or lazy
no broken engine mounts
if nothing obvious

i would have a look at the back bulbs, holder and earths

check the linkages gear stick to gearbox.

it should be by the driver side firewall under hood

Speed sensor sends a signal to the on-board computer reporting the vehicle speed

There is a speed sensor for the transmission inside the speedometer gauge.
if the speedometer cable is broken the hold light will blink. indicating a trans
malfunction code and the transmission will not work properly.
do not drive too long without a speedometer cable.

Probable culprit would be the electric solenoid that releases the gear shift linkage when you apply the brake pedal. Since it's electric, a dead battery will cause the same result. Check to see if your brake lights are working. If not try replacing your brake light switch as this also gets power to the above mentioned solenoid.

This is a safety feature to prevent children from accidentally putting a car in gear.

Good luck!

Click button on auto shift to turn off, it means it is is held in gear

When it does start going if the RPMs are raising but will not shift then most likely the transmission is slipping and needs to be rebuilt. If one or more shift solenoids are stuck/not working than it will only stay in one gear and will not shift. Most likely you are looking at a new or rebuilt transmission.

This is a known issue with certain '03 and '04 MPV transmissions; my 2003 had the same problem. There is a Mazda technical service bulletin about this - 05-003/004 (Shift Shock). You need to take the van to a qualified shop and have them flash the transmission control module (TCM) with new firmware.

I had this done. It did not completely fix the problem but it did reduce its occurrence.