Starting Problem

[NoMechanicHere]

Hi everyone. First post here!

Ok, so I've got a Merc 4.3LX/V6 with a BRAND NEW deep cycle marine battery. Went to start her for the first time this year and when I turned the key all I heard was one click - determined the click was coming from the slave solenoid. Tried bypassing the main starter solenoid and got the starter to spin but it wouldn't engage the flywheel. So then I jumped the slave solenoid by connecting the main posts with a screw driver and it did turn the starter AND engaged the flywheel. Thought It was the slave solenoid so I replaced it yesterday and I'm still getting the single click when I turn the key.

Here's the one kicker.... if I disconnect the safety lanyard and plug it back in I can get it to turn over about 1 out of 20 times. It's probably worth stating that when I do get it started the engine runs great.

Here's what I have done so far:
-Replaced battery and confirmed it is fully charged
-filed all connections to the battery, no corrosion on cables
-replaced slave solenoid and filed connectors down
-Checked fuses
-visually checked for crimps in cables

It's probably unrelated but I can't get my stereo amp to power on either. CD player works, but since amp won't power on I have no audio. Checked all fuses and they are good.

Any suggestion as to why it clicks when I turn the key would be much appreciated.