I can't open the backdoor for some reason I tried fixing it like how it said on the manual that came with it but it didn't work

It has a Magic Hole on the inside. Seems to be a known issue on them.

https://www.youtube.com/watch?v=Xy2LuGS2ytw

https://www.google.com/search?q=Blazer+%2799+Backdoor+won%27t+open

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Hi. I'd like to know what you are working on (Year, Make and Model). I think you are expressing that the side rear doors and backdoors wont unlock. If this is the problem diagnostics will need to be performed to find the source. Fuses, relay, master control switch, control module or wiring could be the problem. Please let me know how you want to proceed. Good Luck.

see if you can take the door panel off from the inside and open manually and change the lock or call a lock smith

i have the same problem on my Merc C200k coupe 2002 i have been told you have to go through the grill but i cant work out how to get the gill off with out opening the bonnet lol

sounds like you were running it without oil..that would explain why there was no oil leaking on the ground..the gearbox would now be worn out so it's best to get another trans,, could be a rebuilt or from a scrapped crashed vehicle..if the trans was running out of oil, best not try to repair it..

Both doors? Did it happen simultaneously or at different times?

it sounds to me like the linkage inside the door may have come off. you have to remove the door panel that is if you can open the door from inside. if not you may be able to open the door with a slim jim. no matter what you have to get the door panel off to fix your problem. that can be a task if you can't open the door.

Open the door and spray some white lithium grease into the latch mech. if that doesnt work try checking the latch cable for damage or stretching

replace the hatch struts. I'm assuming you're talking about the back hatch. There should be 1 or 2 struts (look like shock asborbers), they've probably gone bad.