low volt issue & then 3000 rpm when put in prk

so I had replaced the intake gasket(s) a yr ago, and forgot to plug in 1 wire harness into the alternator at the back, and when I started it up, headlights were dim, battery discharged, and the rpm's were @ 3000 when in park ? so I came home Thurs night, and was ready to park my car,and it started acting the same way as then... I checked that same wire connection, and it was locked in the alternator, so why would it start acting like that now ?? any ideas

Check the charging rate for starters, any check engine lite on?