The black wire should connect to the battery POSITIVE terminal. The white wire should connect to the battery NEGATIVE terminal. (Edited per RoyB's correction!)
Before you connect the battery:
0. UNPLUG/DISCONNECT THE TRAILER FROM ANY AND ALL ELECTRICAL SOURCES!!!
1. Use a multimeter set to measure resistance (Ohms - the symbol looks like a horseshoe), set at the lowest value possible.
2. Put the two leads together to verify a ZERO or near zero reading.
3. Find a good point on the frame to touch the meter lead#1 to (it doesn't matter which one for this test). The frame point shouldn't have any paint, rust, mud; it should be nice clean metal. Hold it there during the following steps.
4. Touch the meter lead#2 to the WHITE wire's conductor. The meter should read ZERO or pretty close to it.
5. Touch the meter lead#2 to the BLACK wire's connector. The meter should read a really high value (or display no value at all).
This should verify that which wire is connected to the trailer's frame and will connect to the negative battery terminal.
If this is not the case, post back for further advice.
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