99 sierra shifts rough only in 1st gear and tachometer doesn't work

I have a 1999 GMC Sierra. I replaced all the spark
plugs and fuel pump. I left and ignition coil on the
back loose so it was idling rough. I plugged it in
and ever since then I wasn't able to crank it. My
mechanic was able to bypass something by
putting a fuel fuse switch underneath the hood. I
have to turn it on and off accordingly to start the
vehicle. But it's still rides rough in first gear only
and the tachometer does not work at all. Took it to
a dealer and they said the ECM was bad. I already
had a new ECM put in and flashed so I knew that
was incorrect information. And they also said the
transmission was bad. I also knew that was
incorrect information because it only started after i
connected the last ignition coil.  I heard replacing
the ignition switch would solve the issue' but i dobt
want to keep throwong more parts at it..Any help
would be appreciated thank you.