Where is the $_params field of the CRM_Contribute_Form_ContributionBase class set?
I highly recommend setting up an IDE with debugger so you can easily step through the code while it is executing. Yes it's a little bit of work to install and learn to use those tools, but worth it!

My IDE tells me that the $_params variable is an array declared in CRM_Contribute_Form_ContributionBase but mostly populated during CRM_Contribute_Form_Contribution_Confirm::preProcess().