Black screen from liquid, works with external monitor.

Hey,

I spilled a small amount of water (~tsp) into the back air vent of my 2014 macbook pro retina while it was closed. I flipped it upside down, wiped it off, and dried it thoroughly with a room dehumidifier. The computer now powers up normally but to a black screen. It works perfectly with an eternal monitor via hdmi. I tried the various software black screen fixes (reseting PRAM, brightness, ect) but nothing helped.

It seems to me like the most likely problem is just the cable that transmits the signal to the LCD is burnt out/not functioning. I brought it to a laptop repair place (laptopMD) and told them all this but they were not very helpful. Paraphrased: "We have to take it apart and look at it ourselves to know what's wrong. For water damage base charge is $500 minimum + the cost of any parts + tax" I declined to let them have it (not paying over $500 to have someone look at it when it could be a simple fix!)

What's the next step I should take? Is it worth bringing to the apple store? (It is no longer under warranty) I hear it's basically the same deal where they don't do in-house repair and it's some ridiculous minimum charge for water damage because they just replace the whole logic board without troubleshooting the precise problem. Is that true?

I also could not find a guide on how to replace the cable that connects the display, is there one? All the ones I saw were to replace the screen itself, but that shouldn't be a problem since no water got into the upper clamshell and there was no physical trama to the screen. Not that I am overly confident in repairing it myself.

Any advice on the matter is greatly appreciated!

Thanks

- greg

****UPDATE****

It's the backlight. Can see the screen with a powerful flashlight. How to fix?