Making payment on the first after TS

My partner and I bought a 2nd and subsequently foreclosed on it through TS. Given the terms of the first mortgage (30 yr fix at 3%, $1,300 a month, still current) and the going rent at $3,000/mo, we are thinking about doing a wrap to sell it. My question is: how do I make payments (by me or the end buyer) on the first (for the long term, not just a couple of months) when the lender of the 1st mortgage doesn't disclose any info. I could guess based on the current monthly payment info which I have, but I am hoping to find a better way. Any idea?

Lin