I havent used a Tezet system, however photos of the wiring diagrams are on their ebay listings. AKU looks like it goes to battery positive.
More than likely the regulator has the same wiring as a standard universal 6-wire regulator / rectifier. If this is the case, it’ll have 3 phase inputs (3 yellow wires), a ground, battery positive and a sensing wire. So I’m pretty sure, from a google search of the universal regulators, that your remaining red wire is to battery positive. Also, translating battery into polish gives “akumulator”.
If in doubt – email tezet directly. They seem very helpful on facebook