[ViragoJoe]

Please help me I am stuck.

The cable shown in the photos goes straight into the rectifier also shown in the photos.

it is a ring terminal but I have no clue which battery terminal to connect it to (positive or negative)

motorcycle: 1987 Yamaha Virago 535

I can’t make sense of the wire diagram can someone please help? View attachment 3697
View attachment 3696
View attachment 3695

enigma (a.k.a. mysterious),
Don't know if this '87 XV535 was been modified in anyway. But your Reg/Rec isn't OEM standard design.
Your Rectifier & Regulator Assembly is 42X-81960-A1-00.

New Voltage Regulator Rectifier for Yamaha Virago 42X-81960-A1-00 42X-81960-A1 | eBay


However, if that black wire is coming from the "Current" Reg/Rec. Then it goes to the Negative terminal on the battery.

Your wining schematic is for a U.S. model XV535, may be different for Canadian spec'd. model. The schematic top right is the wiring for the battery.

Lets start with the Negative/Ground connection:
There should be be a Heavy Black Cable going from the battery to the frame and/or the engine for grounding purposes.

Then your Lighter Gauge Black Wire connects lighting and all other components to ground on battery. Based upon your wiring schematic.

Now for the Positive Battery Connection. There is a Light Gauge Red Wire that comes from the Regulator/Rectifier that charges the battery. Work backwards from the Regulator/Rectifier and you will see how it connects to the battery via the fuse box

Although the schematic shows a Heavy Black Wire going to the Battery POSITIVE POST from the Starter Relay (Solenoid) it is indeed a POSITIVE Connection.
Somewhere along the routing it may become red as it gets closer to the battery. Why this is, I have no idea! On my Virago at the Solenoid that wire is black, however, at the battery post it is RED. Go figure.

Hope this clarifies the schematic.