A1278 No power to HD

Ok so here is the skinny:
Had a lady bring in a Macbook Pro 13" Mid 2012 that couldn't boot to the OS. Originally I suspected a dead HD, but after testing the hd in a pc we use to test hard drives in (using bootable HD test, Hirens) I found the drive to be functioning fine. On a hunch I hooked the HD up a USB dock and booted fine off of the and was able to access all her files. This lead me to believe that the HD cable must have gone bad, found a few posts that seem to confirm this theory and ordered her a replacement sata cable (821-1480-A).
Here is where it gets unfortunate; I connected the replacement cable and attempted to boot it internally, got the same result as before; no drive found. This is leading me to believe there is an issue with the motherboard itself. I've listened to the HD and there is noise on boot up from it and no vibration.
Anyone been in this situation before or no of anything else I can try?

Just out of curiosity - would there be a chance anything got spilled on it? I will assume you have already replaced the cable correctly which can go bad and be a bit trick to replace. A lack of power makes me wonder if there has been a short. Liquid would do that. The link below shows where the internal indicators are.

http://appleinsider.com/articles/08/10/15/inside_the_new_macbooks_lsi_battery_hd_and_ram.html

Sounds like you did some great troubleshooting by testing the HD in another computer...and great testing by replacing the HD data cable in the MacBook Pro. Assuming that the new HD cable is good...agree with Lisa...very likely a logic board issue. I guess if you could test that new HD data cable in some way (check for continuity on each of the data cable wires...or in a 2nd MacBook Pro)...this would confirm that the new cable is good/bad...and lead back to the logic board of the MacBook Pro as the issue.

- Nick

Maybe give an NVRAM, or SMC reset, a try?

https://support.apple.com/en-us/HT204063
https://support.apple.com/en-us/HT201295

I gave the indicators a once over and they are all white, so it doesn't look like liquid damage. Gave the NVRAM and SMC reset a try, no luck there unfortunately.
I'm going to do a search for one but by any chance do any of you have a pin out or wiring diagram for that SATA cable? Unfortunately hers is the only macbook I have in the shop at the moment, so I can't do any part swaps.

Seems to me I came across another macbook awhile ago that was having powering on issues and it ended up being a microfuse had blown on the motherboard. Any chance I'm looking at a similar issue?