Easy route: Start with a current vendor statement.
Work from there...if it's not on the current statement, than it needs to be adjusted off.
Your prior year P&L's are already incorrect, no point in trying to fix it....unless you're going to restate your prior year financials.
Hard route: Get an account history from the vendor for each of the prior years and reconcile.
Assets = Liabilities + Equity (which includes Current YTD Net Earnings or the "P&L")
Check your postings to stat accounts and/or other accounts that are not normally in the P&L and Balance Sheet.
If you post something against a stat, note that it's not an Asset, Liability, Equity, Revenue or Expense....that can throw off your balance.