The factory pump wiring is small and made from aluminum. It was marginal from the factory, and, with age, does not do the job when combined with larger pumps and increased demand. The average Walbro increases 15-17% in flow for each one volt increase at the pump. That is a lot.
Commercially made hot wire kits are available from a number of vendors.
Alternatively, you can make your own kit if you have access to a crimp on connector tool, or, a soldering iron.
All you need is some 10 gauge wire, a HD inline fuse holder and a 30 amp fuse plus a 30 amp name brand relay and some crimp on connectors (I always solder the connectors after crimping).
Mount the relay on the driver's side inner fender next to the fan relays. Run a wire from the the back of the alternator, through the 30 amp fuse and on to the input power terminal (30) on the relay. Run another wire from the output power terminal (87) on the relay along the inner fender, down the firewall, and into the frame rail. Pull this wire through the frame and out the rear end. Run it up and over the top of the fuel tank using tie wraps to keep it from rubbing any moving suspension pieces. Cut the fuel tank power feed wire on the tank side of the factory WeatherPak connector and connect the new wire to the wire going to the pump. I always put a connector at this point so I can disconnect the hot wire and reconnect the factory wire as a back up. I have never had a failure but, the back up might come in handy one day.
Now, go back to the new relay you mounted and make a ground wire that goes from the ground terminal (85) on the relay and then to the inner fender sheet metal. This can be a short wire that loops back to the screw that mounts the relay to the inner fender.
That leaves one wire to be connected. Run a wire from the fuel pump test connector just behind and below the alternator to the trigger terminal (86) of the new relay. This allows the normal two second activation of the the relay when starting the engine. That's all it takes.
You can make this as simple as possible by buying a relay and harness connector harness together. If you wish to add weather proof connectors, etc, that is your option.
After installing the hot wire, take your meter and measure the voltage drop from the tank to the factory sheet metal (not the frame). the engine needs to be running. Clean off a spot by the tank strap pivot on the underside of the trunk floor. Set the meter to volts and put one probe on the tank and one on the spot you just cleaned off....don't be surprised to find several tenths of a volt on the meter readout. This indicates the voltage being lost due to the poor factory ground. Do not use the frame, itself, as it is not a direct ground on the Regal.
Go to the folded up corner of the tank, and, drill a hole as close to the edge of the flap as possible. The trick here is to be outside the pinch weld that joins the two tank halves together. Otherwise you will be sealing a slow drip if the hole is inboard to the seam! Drill another hole in the tank strap bracket and install a ground strap, or a piece of 10-12 gauge wire between the two holes...metal screws will hold the two end connectors. With the engine running, recheck the voltage drop...it should be less than one tenth of a volt now.