I just got done modding a hori V3 fightstick to a madcatz brawlpad. At first, everything seemed to work fine until i put the case back on then after a few seconds, the top row (all four buttons) seemed to stop working. Strange thing is… if i remap the buttons in the street fighter configuration menu, some Of these very same buttons will now work. I think every button works (getting signal) but only when i remap them… but it cancels out other buttons. This is the case for both systems when i Tested them. Im using a dpdt switch. plugged into my pc To check controller properties, button 5 is constantly lit. Also, two buttons seem to be the x and y axis. Are my usb dAta cables touching? Anyone has any idea? Sorry for sloppiness, sent from phone.
Button properties in the brawlpad on PC are as follows (if i remember right)
Xbox - PS3
1 - A - Square
2 - B - X
3 - X - Circle
4 - Y - Triangle
5 - LB - L1
6 - RB - R1
7 - LT - L2
8 - RT - R2
9 - ST
10 - SL
11 - L3
12 - R3
13 - Home
check if you’ve grounded the L1 on the v3 side
7 and 8 on a brawlpad/ms stick activate the top slider on the axis
It shouldn’t be your data cables because if it was touching, you wouldn’t even have your controller be read.
See if your solder leads going to the buttons are too long as some inputs shouldn’t register if they are already pressed as on
If you remember right? :-/
I’ve looked quite a few times for this a few times over the last 6 months. Thank you. 
Thanks Mr mortified! Quick response. It was exactly as you said…checked it last night with multimeter and found the LB button (5 on pc)was grounded. This was because I messed up the copper pretty bad And had to solder onto the thin trace. When scraping the trace, I also exposed some of the light green area next to the dark green divider lines, thus my wire was actually touching both parts. I didn’t realize the light green Background of the board was all ground??
I had to scrap the trace just a bit further down and finally got it working right!! It was hard to tell because more than one button was not working. What an experience this was…My first successful dual mod. (Pats self on back) thanks again dude!