I recently bought a 7" kkmoon wifi folding dashcam but the rearview camera is broken, can i buy just the rearview camera if so what is cost, apart from that dashcam is OK.

Why do you think you need to update Firmware?

Ask them direct. There are no download sites and they don't offer it. It is Budget price Chinese junk. These DVR and Camera Bundles are a one-time offer and they move on to the next iteration.

https://www.kkmoon.com/home-improvement-2930/

https://www.google.com/search?q=kkmoon+dvr+firmware+download

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Reset probably

https://www.google.com/search?q=kkmoon+surveillance+cameras+reset

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Up to 64 ft wireless transmission
Up to 99 ft infrared camera vision
Full details can be found here:
https://www.kkmoon.com/p-s441-us.html

Circuit/System Description
When the transmission is placed into REVERSE, a 12 volt signal is sent to the rearview camera image display module indicating that camera operation is requested. The rearview camera image display module then supplies ignition voltage and ground to the rear vision camera. The rear vision camera sends video information through the rearview camera image display module to the inside rearview mirror through a video signal + and a video signal - circuit. A grounded shielding also wraps the video signal circuits to reduce electronic interference which may degrade the video signal and cause a distorted or otherwise degraded image.

Rearview Camera Image Display Module (UVC)
In the instrument panel behind the driver side knee bolster to the right of the steering column
Do you know how to do electrical testing ?

Rear Vision Camera System Does Not Display
Ignition OFF, disconnect the harness connector at the rear view camera image display module.
Test for less than 1.0 ohm between the ground circuit terminal 3 and ground.
? If greater than the specified range, test the ground circuit for an open/high resistance.
Ignition ON, verify that a test lamp illuminates between the ignition circuit terminal 2 and ground.
? If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance.
Ignition ON, park brake applied, vehicle in REVERSE, verify that a test lamp illuminates between the control circuit terminal 1 and ground.
? If the test lamp does not illuminate, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.
Ignition OFF, connect the harness connector at the rear view camera image display module. Disconnect the harness connector at the rearview camera.
Ignition OFF, test for less than 1.0 ohm between the ground circuit terminal B and ground.
? If greater than the specified range, test the ground circuit for an open/high resistance. If the circuit tests normal, replace the rear view camera image display module.
Ignition ON, park brake applied, vehicle in REVERSE, test for 5.8-6.2 volts between the 6-volt reference circuit terminal A and ground.
? If less than the specified range, test the 6-volt reference circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the rear view camera image display module.
? If greater than the specified range, test the 6-volt reference circuit for a short to voltage. If the circuit tests normal, replace the rear view camera image display module.
Ignition OFF, disconnect the harness connector at the rear view camera image display module.
Test for less then 1 ohm between the shield ground circuit terminal 10 at the rear vision camera display module harness connector and terminal D at the rear vision camera harness connector.
? If greater than the specified range, test the shield ground circuit for an open/high resistance.
Ignition ON, test for less than 1 volt between the signal circuit terminal 9 and ground.
? If greater than the specified range, test the signal circuit for a short to voltage.
Test for infinite resistance between the signal circuit terminal 9 and ground.
? If not the specified value, test the signal circuit for a short to ground.
Test for less then 1 ohm between the signal circuit terminal 9 at the rear vision camera display module harness connector and terminal C at the rear vision camera harness connector.
? If greater than the specified range, test the signal circuit for an open/high resistance.
Ignition OFF, disconnect the harness connector at the inside rearview mirror.
Ignition ON, test for less than 1 volt between the following signal circuits at the inside rearview mirror harness connector and ground:
• Video + signal circuit terminal 6
• Video - signal circuit terminal 7
Test for infinite resistance between the following signal circuits at the inside rearview mirror harness connector and ground:
? If greater than the specified range, test the signal circuit for a short to voltage.
• Video + signal circuit terminal 6
• Video - signal circuit terminal 7
? If greater than the specified value, test the signal circuit for a short to ground.
Test for less then 1 ohm between the following signal circuits:
• Signal circuit terminal 12 at the rearview camera image module and terminal 6 at the inside rearview mirror.
• Signal circuit terminal 11 at the rearview camera image module and terminal 7 at the inside rearview mirror.
? If greater than the specified range, test the signal circuit for an open/high resistance.
If all circuits test normal, test or replace the rear vision camera.
Ignition ON, park brake applied, vehicle in REVERSE, verify a clear rear vision image is displayed on the radio.
? If a clear rear vision image is not displayed, test or replace the inside rearview mirror

It is between $70-$100, depends on the manufucturer and on the on-line store you are buying from.

From your description you would be further ahead to buy a new camera, you stated there are no repair shops where you live, and to send it away would be expensive probably costing more than a new one. The only other option is go on ebay to price a new screen, but you will need the skills to take it apart and soldering may be required to replace the panel display.

I´ve got the same issue. If you use the batteries that were in A540 in another camera, does it still work?

The part can be bought on ebay or even craigs list, Many junkyards have them too a good price is $20 but their just under $100 new

Sony has bought-out Minolta. Parts for Minolta equipment has become scarce, but try here: http://eservice.sony.com/webrma/web/index.do